Within the printing industry there is a variety of techniques known for transferring designs to paper, for example, by means of print originals. One possibility is that known as flexographic printing. One embodiment of flexographic printing, in turn, is the use of multi-layered photopolymer printing plates having a flexible substructure, this type of printing having been part of the prior art for some considerable time. These printing plates are composed of a plurality of layers of different polymeric materials each with specific functions. For example, the “Nyloflex ACE” printing plates from the Flint Group company have at least two layers, namely a light-sensitive relief layer and, beneath it, a stabilizing film.
In the flexographic printing process, flexible printing plates are bonded to printing cylinders. This adhesive bonding is generally carried out using double-sided pressure-sensitive adhesive tapes (PSA tapes), on which very stringent requirements are imposed. For the printing process, the PSA tape is required to have a certain hardness but also a certain elasticity. These properties must be set very precisely in order that the printed image produced yields the desired result in accordance with the requirements. Further stringent requirements are imposed on the PSA, where the bond strength must likewise be sufficiently high that the printing plate does not detach from the double-sided PSA tape, or the PSA tape from the cylinder. This must be so even at elevated temperatures of 40 to 60° C. and at relatively high printing speeds. In addition to this property, however, the PSA is also required to possess reversible adhesion properties, since frequently it is necessary to bond printing plates and then detach them again for repositioning. This detachability ought to exist even for an adhesive bond over a relatively long period of time (up to 6 months). Moreover, it is desired that the PSA tape and especially the printing plate can be removed again without their destruction, i.e. without great application of force, since usually the printing plates are used a number of times. Furthermore, there should be no residues on the printing plate or on the cylinder. In summary, then, very stringent requirements are imposed on the double-sided PSA tapes that are suitable for this utility.
U.S. Pat. No. 4,380,956 A describes a process for fixing a printing plate for the flexographic printing process. PSAs are used for that process too, but have not been specified in any greater detail.
GB 1,533,431 A claims a double-sided PSA tape including an elastomeric layer which in turn has been foamed by fragile air bubbles. The air bubbles are destroyed under pressure during the flexographic printing application.
U.S. Pat. No. 4,574,697 A claims double-sided PSA tapes comprising as their carrier material a flexible polyurethane foam affixed to a PET (polyethylene terephthalate) film. The external layers are composed of PSAs. The PSA tape described is said to be reversible and to be removable from the printing cylinder and from the printing plate. A similar product structure has been described in EP 0 206 760 A. There the flexible foam carrier used was a polyethylene foam.
U.S. Pat. No. 4,574,712 A describes, in analogy to U.S. Pat. No. 4,574,697 A, a similar PSA tape construction. Here there is a restriction on the PSAs to the effect that the bond strength to the printing plate and to the printing cylinder should be lower than to the carrier film and the carrier foam.
U.S. Pat. No. 3,983,287 A describes a laminate whose carrier material comprises an incompressible elastomer. Compressibility is achieved by means of beads which are destroyed under pressure and which therefore produce flexibility.
U.S. Pat. No. 5,613,942 A describes PSA tapes which are especially suitable for bonds on wet surfaces. It is also said that such PSA tapes are suitable for bonding printing plates.
U.S. Pat. No. 5,476,712 A likewise describes a double-sided PSA tape which is used in the flexographic printing process. This PSA tape comprises, in turn, a thermoplastic elastomer, the structure present in this case being a cellular structure produced by means of expanding microparticles.
EP 1 590 383 A describes PSA tapes for bonding printing plates, and processes for producing them. The invention relates to PSA tapes and to a process for preparing PSAs for bonding printing plates, the PSA possessing a very low level of peel increase on polar surfaces, being readily repositionable and, after bonding to the printing cylinder, being said to exhibit little edge lifting of the printing plate and of the plate-mounting tape/printing plate assembly.
In the cases referred to above, a very large number of different PSAs are employed. Natural rubber adhesives possess good tack properties but lack great shear strength at room temperature and age as a result of degradation via the double bonds present in the polymer. SIS-based or SEBS-based PSAs are generally very soft and tacky and tend to soften at high temperatures as well. If the printing plate is bonded to the printing cylinder under tension using an SIS or SEBS PSA, the printing plate tends to detach, despite the fact that the bond strength is high. Acrylate PSAs, in contrast, are more suitable for bonding printing plates to printing cylinders, but have to be crosslinked in the preparation process after the coating operation. Moreover, as a result of the large number of ester groups and the resulting polarity, these PSAs have a tendency towards a high level of peel increase. As a result, the printing plates can be removed only with very high application of force. Moreover, the PSA must have a certain hardness in order that the printing plates, after bonding to the printing cylinder, do not tend towards edge lifting over a prolonged time period.
The phenomenon of edge lifting is a problem which is known for virtually all current commercial adhesive printing plate tapes, and results from the fact that the flexible printing plates, after the operation of bonding to a printing cylinder—and the deformation this inevitably involves—exhibit a propensity to resile into the planar area. The propensity towards edge lifting is dependent on parameters which include the cylinder diameter, the thickness and nature of the printing plate, the presence of a printing plate margin which is not used for printing (and which in that case is thinner and exhibits lower resilience forces), on the print design at the plate margin (if the printing plate is printed over the full area it typically has the highest resilience forces), on the cleaning of the reverse of the printing plate prior to mounting on the cylinder, on the pressure applied to the plates in the course of bonding, and on further parameters. Where two or more printing plates are used simultaneously, the requirements imposed on the gap between the printing plates, which are typically bonded seamlessly, become ever greater, particularly if large-format prints are to be produced in which the transition is to be invisible. Flexographic printing (i.e. printing using printing plates) is increasingly competing with other processes such as that of gravure printing, for example, and so the aforementioned requirements are gaining in significance. Besides the fact that the gap must be increasingly small (less visible for the printed image), there is also no longer a possibility of leaving a non-print margin. Printers are attempting to master these problems at present by carrying out printing processes with two or more worksteps, but this entails considerable inconvenience and additional costs.
It is an object of the invention, therefore to offer a pressure-sensitive adhesive which is outstandingly suitable for the bonding of flexible printing plates, and also to provide a pressure-sensitive adhesive tape featuring such a pressure-sensitive adhesive, so that even after prolonged storage, under slightly elevated temperatures there is no edge lifting or at least greatly reduced edge lifting, when a flexible printing plate is bonded to a printing cylinder using such a pressure-sensitive adhesive tape.